Clock Jitter Emulation

ABSTRACT

An emulator emulating a DUT emulates a clock generator for generating clock signals of the DUT with jitter. As part of generating clock signals, the emulator generates a jitter clock value for each clock signal. To generate a jitter clock value for a clock signal, the emulator identifies a clock parameter stored for the clock signal and sums the clock parameter with a jitter value randomly selected from the jitter range of the clock signal. When a system fast clock cycle starts, the emulator determines the lowest value from the generated jitter clock values. The emulator outputs an edge on clock signal having the lowest jitter clock value. The emulator generates a new jitter clock value for each clock signal and the process repeats during the next system fast clock cycle.

BACKGROUND

1. Field of Art

The disclosure generally relates to the emulation of designs, and morespecifically to emulating clocks with jitter.

2. Description of the Related Art

Emulators have been developed to assist circuit designers in designingand debugging highly complex integrated circuit designs (for example, asystem on a chip (SOC)). An emulator includes multiple fieldprogrammable gate arrays (FPGAs) or specific programmable emulationcomponents that together can imitate the operations of an integratedcircuit. By using an emulator to imitate the operations of an integratedcircuit, designers can verify that the integrated circuit complies withvarious design requirements prior to fabrication.

An integrated circuit typically includes multiple components thatoperate on different clocks. Hence, the integrated circuit includesmultiple clock domains. Signals routinely pass between different clockdomains. During clock domain crossings, errors/anomalies may beintroduced into the integrated circuit. One source of the errors couldbe the jitter present in the clock signals. For example, two clocks maybe designed to operate at 1 GHz. However, because of jitter thelikelihood that both clocks will be aligned (both operating at 1 GHz) isvery small. Hence, when signals cross the clock domains, errors mayoccur because the clocks are not operating exactly as expected.

When an emulator emulates an integrated circuit, the clocks in theemulation environment are perfect clocks in that they start and end asdesigned (do not include jitter). Since the emulated clocks are nottaking jitter into account, certain errors caused by jitter may not bedetected when emulating the integrated circuit.

BRIEF DESCRIPTION OF DRAWINGS

The disclosed embodiments have other advantages and features which willbe more readily apparent from the detailed description, the appendedclaims, and the accompanying figures (or drawings). A brief introductionof the figures is below.

FIG. 1 is a block diagram of an emulation environment, according to oneembodiment.

FIG. 2 is a block diagram illustrating a host system, according to oneembodiment.

FIG. 3 is a block diagram illustrating a clock generator emulated by anemulator to generate clock signals, according to one embodiment.

FIG. 4 is a timing diagram illustrating clock signals and a system fastclock, according to one embodiment.

FIG. 5 is a flow chart illustrating a process of emulating multipleclock signals, according to one embodiment.

FIG. 6 is a block diagram illustrating components of an example machineable to read instructions from a machine-readable medium and executethem in a processor, according to one embodiment.

DETAILED DESCRIPTION

The Figures (FIGS.) and the following description describe certainembodiments by way of illustration only. One skilled in the art willreadily recognize from the following description that alternativeembodiments of the structures and methods illustrated herein may beemployed without departing from the principles described herein.

Reference will now be made to several embodiments, examples of which areillustrated in the accompanying figures. The figures use like referencenumerals to identify like elements. A letter after a reference numeral,such as “302A,” indicates that the text refers specifically to theelement having that particular reference numeral. A reference numeral inthe text without a following letter, such as “302,” refers to any or allof the elements in the figures bearing that reference numeral.

Overview

A disclosed system (method and computer program product) are describedfor emulating clocks of a design under test (DUT) with jitter. Anemulator emulates a clock generator to generate clock signals withjitter for a DUT being emulated. The clock generator includes for eachclock signal a random jitter generator, an accumulator and a comparator.The clock generator also includes a minimum operator.

Each random jitter generator reads from a memory a clock parameterassociated with the respective clock signal. The clock parameter may be,for example, a width of the positive portion (between the rising edgeand falling edge) or negative portion (between the falling edge and thenext rising edge) of the clock's period. A parameterized random jittergenerator generates a jitter clock value for the clock signal based onthe read clock parameter and a jitter value randomly selected from ajitter range (e.g., sum the clock parameter and the randomly selectedjitter value). The random jitter generator stores the jitter clock valuein the respective accumulator.

During a clock cycle of a system fast clock, the minimum operatorobtains the jitter clock values stored by the accumulators. The systemfast clock operates at a higher frequency than the clocks signalsgenerated by the clock generator. The minimum operator identifies fromthe jitter clock values the lowest jitter clock value. The minimumoperator outputs the lowest jitter clock value to each of thecomparators. Each comparator compares the lowest jitter clock value tothe jitter clock value stored by the respective accumulator. If thevalues are not the same, the comparator does not change the respectiveclock signal. For example, if the clock signal was at a high state, theclock signal is maintained at the high state. Further, the comparatordetermines the difference between the jitter clock value stored by theaccumulator and the lowest jitter clock value. The comparator storesdetermined difference in the accumulator as a new jitter clock valuereplacing the value previously stored by the accumulator.

On the other hand, if the values are the same, the comparator causes anedge to be output on the respective clock signal (comparator causes theclock signal to change states). In one embodiment, if the clock signalwas at a low state, the comparator outputs a rising edge. However, ifthe clock signal was at a high state, the comparator outputs a fallingedge. The respective random jitter generator generates a new jitterclock value for the clock signal based on a clock parameter read fromthe memory and a new randomly selected jitter value. The random jittergenerator stores the generated jitter clock value in the correspondingaccumulator replacing the value previously stored by the accumulator.

During each subsequent system fast clock cycle, the process is repeatedof the minimum operator selecting the lowest jitter clock value from thevalues stored by the accumulators, the comparators determining whetherto output an edge for their respective clock signals, and new valuesbeing stored in the accumulators. By incorporating jitter values intothe clock parameters used to generate the clock signals, the cycles ofthe clock signals will vary (not all be the same) which is how clocksoperate in a real world environment. Further, by incorporating jitterand having the clock signals vary, it is possible to detect, forexample, errors that occur during clock domain crossings in the DUT.

Overview Emulation Environment

FIG. 1 is a block diagram illustrating an emulation environment 100,according to one embodiment. The emulation environment 100 includes anemulator 110 and a host system 120. The emulator 110 and the host system120 communicate through an interface 115.

The interface 115 is a communication medium that allows communicationbetween the host system 120 and the emulator 110. In one embodiment, theinterface 115 is a cable with electrical connections. For example, theinterface 115 may be an USB, ETHERNET, optical, or a custom built cable.In other embodiment, the interface 115 is a wireless communicationmedium or a network. For another example, the interface 115 may be awireless communication medium employing a Bluetooth® or IEEE 802.11protocol.

The emulator 110 is a hardware system that emulates designs under test(DUTs). A DUT is one or more circuit designs emulated by the emulator110. The emulator 110 includes FPGAs that can be configured tocollectively emulate a DUT. In other embodiments, the emulator 110includes other types of reconfigurable hardware components instead ofFPGAs. For a DUT that is to be emulated, the emulator 110 receives fromthe host system 120 bit streams (e.g., one or more binary files)including a description of a DUT with one or more clock generators thatgenerate clock signals with jitter for the DUT during emulation.Additionally, the bit stream describes partitions of the DUT created bythe host system 120, mappings of the partitions to emulator FPGAs,placement of logic (DUT logic and clock generator logic) on FPGAs, androutings between placed logic. Based on the bit streams, the emulator110 configures the appropriate FPGAs and emulates the DUT.

The host system 120 configures the emulator 110 for emulating a DUT. Thehost system 120 may be a single computer or a collection of multiplecomputers. In the embodiment where the host system 120 is comprised ofmultiple computers, the functions described herein as being performed bythe host system 120 may be distributed among the multiple computers.

The host system 120 receives from a user a description of a DUT to beimplemented on the emulator 110. In one embodiment, the description ofthe DUT is in a type of hardware description language (HDL), such asregister transfer language (RTL). The host system 120 creates a gatelevel netlist based on the HDL description of the DUT. In anotherembodiment, the description of the DUT received from the user is in agate level netlist. The host system 120 incorporates into the DUT one ormore clock generators. Each clock generator generates clock signals ofthe DUT with jitter.

The host system 120 generates one or more bit streams which includeinformation to configure the emulator FPGAs to emulate the DUT with theclock generators. A bit stream may include, for example, a designdescription of one or more partitions of the DUT, mapping information(e.g., mappings of partitions to FPGAs), placement and routinginformation, and design constraints for the DUT.

Through interface 115, the host system 120 transmits to the emulator 110the created bit streams to configure the FPGAs to emulate the DUT.During and/or after the emulator 110 emulates the DUT, the host system120 receives emulation results from the emulator 110. Emulation resultsare information generated by the emulator 110 based on the emulation ofthe DUT.

FIG. 2 is a block diagram illustrating the host system 120 in moredetail, according to one embodiment. The host system 120 includes aninput receiver 200, synthesizer 210, logical mapping module 220,partitioning module 230, technology mapping module 240, placing androuting module 250, and bit stream generation module 260. Each of thesecomponents may be embodied as hardware, software, firmware, or acombination thereof. Together these components generate information toconfigure the emulator 110 to emulate a DUT.

The input receiver 200 receives descriptions of DUTs to be implementedby the emulator 110. In one embodiment, the input receiver 200 receivesa description of a DUT in HDL or in a gate level netlist.

The synthesizer 210 converts HDL descriptions into gate level logic. Ifa description of the DUT is received in HDL, the synthesizer 210synthesizes the HDL description to create a gate level netlist with adescription of the DUT in terms of gate level logic. In one embodiment,the synthesizer 210 also converts a received gate level netlist for theDUT into another gate level netlist.

The logical mapping module 220 maps logic of the DUT to componentsavailable in the FPGAs of the emulator 110. The logical mapping module220 identifies logic included in the gate level netlist of the DUT thatis not available in the emulator FPGAs and associates (assigns) acorresponding hardware component that is available in an emulator FPGA.For example, the logical mapping module 220 identifies a Boolean logicgate in the gate level netlist and associates the Boolean logic gatewith a corresponding logic gate or a look up table (LUT) unit availablein an FPGA. In one embodiment, the logical mapping module 220 modifiesthe gate level netlist based on the mapping.

In one embodiment, the logical mapping module 220 also incorporates intothe DUT one or more clock generators that generate the clock signals ofthe DUT. The clock signals generated by a clock generator includejitter. Details of the components and design of a clock generator aredescribed below with reference to FIG. 3.

In one embodiment, the logical mapping module 220 analyzes the DUT andidentifies the clock signals of the DUT. For each set of clock signals,the logical mapping module 220 includes a clock generator. In anotherembodiment, the logical mapping module 220 includes a single clockgenerator for generating all of the DUT's clock signals. The logicalmapping module 220 incorporates the one or more clock generators bymodifying the gate level netlist of the DUT. In another embodiment, theone or more clock generators are incorporated into the DUT prior tosynthesizing the HDL description of the DUT.

The partitioning module 230 partitions the DUT and maps the partitionsto emulator FPGAs. The partitioning module 230 partitions the DUT at thegate level into a number of partitions using the DUT's netlist. Thepartitioning module 230 maps each partition to one or more FPGAs of theemulator 110. The partitioning module 230 performs the partitioning andmapping using design rules, design constraints (e.g., timing or logicconstraints), and information about the emulator 110.

The technology mapping module 240 maps physical components of the DUTbased on the logical mapping and partitioning. Specifically, ifnecessary, the technology mapping module 240 modifies one or morepartitions based on the partitions created and the mappings of thepartitions to the FPGAs. For example, assume the DUT includes threelogic gates where an output of a first logic gate is connected to aninput of a second logic gate and an input of a third logic gate. The DUTmay be partitioned such that the first logic gate and the second logicgate are to be implemented on the same FPGA, but the third logic gate isto be implemented on a different FPGA. A connection between the firstlogic gate and the third logic gate in different FPGAs may have anadditional delay compared to a connection between two logic gates in thesame FPGA, thereby causing incorrect operations. The technology mappingmodule 240 may add delay elements (or buffers) between the two logicgates on the same FPGA to match the delay between the logic gates ondifferent FPGAs.

The placing and routing module 250 receives the gate level netlist andinformation about the partitioning and mapping, and determines placementand connections of each DUT logic component (including clock generatorcomponents). The placing and routing module 250 places the logiccomponents and the debugging logic in a manner that routings between thelogic components and the debugging logic are optimized.

The bit stream generation module 260 generates one or more bit streamsdescribing information about configurations of the FPGAs of the emulator110. The configurations of the FPGAs include placements of hardwarecomponents to be implemented, connections (i.e., routings) betweencomponents and other design information. The bit stream generationmodule 260 transmits the bits streams to the emulator 110 so that theFPGAs of the emulator 110 can be configured for emulating the DUT withthe one or more clock generators. The bit streams may be stored andprovided to the emulator 110 upon receiving instructions to transmit thebit streams.

FIG. 3 is a block diagram illustrating a clock generator 300 emulated bythe emulator 110 to generate clock signals for an emulated DUT. In thisexample, the illustrated clock generator 300 generates two clock signals312A and 312B of the DUT. Only two clocks signals 312 are shown in thisembodiment for purposes of simplicity. However, in other embodiments,the clock generator 300 will generate more than two clock signals 312.The clock generator 300 operates according to a system fast clock 311that operates at a higher frequency than the clocks signals 312. In oneembodiment, during each cycle of the system fast clock 311 at least oneof the clock signals 312 outputs an edge (e.g., rising edge or fallingedge).

The clock generator 300 includes random jitter generators 302A and 302B,accumulators 304A and 304B, minimum operator 306, and comparators 308Aand 308B. Random jitter generator 302A, accumulator 304A, and comparator308A are used to generate clock signal 312A. Random jitter generator302B, accumulator 304B, and comparator 308B are used to generate clocksignal 312B. In other embodiments where the clock generator 300generates additional clock signals 312 (in addition to signals 312A and312B), the clock generator 300 will include an additional random jittergenerator 302, accumulator 304, and comparator 308 for each additionalclock signal.

The random jitter generators 302 incorporate jitter into theirrespective clock signal 312. In one embodiment, each random jittergenerator 302 stores a jitter range (e.g., −N to +M) which representsthe jitter values that the corresponding clock signal 312 can have. Thejitter range is set based on design specification of the correspondingclock signal. For example, the design specification of a clock signal312 may indicate that the clock signal 312 operates at 1 GHz±10% jitter.Based on the ±10%, the jitter range is set. In another embodiment, therandom jitter generator 302 stores more complex jitter values whichallow determining the jitter value based on an equation using thoseparameters.

Through input 314 each random jitter generator 302 reads a clockparameter of the corresponding clock signal 312 from memory 310. In oneembodiment, the memory 310 is separate from the clock generator 300 asshown in FIG. 3. In other embodiments, the memory 310 is part of theclock generator 300. In another embodiment, multiple memories are usedto store the clock parameters. In another embodiment, there is no memoryand the clock parameters are obtained through other means, like acommunication channel.

In one embodiment, each clock parameter stored by the memory 310 isassociated with a period of a clock cycle. In one embodiment, for eachclock signal 312, the memory 310 stores at least two clock parameters.One clock parameter is the width of the positive portion of the clock'speriod, which is between the rising edge and the falling edge. The otherclock parameter is the width of the negative portion of the clock'speriod, which is between the falling edge and the next rising edge. Therandom jitter generator 302 reads at least one of the clock parametersof its respective clock signal 312. In another embodiment, for eachclock signal 312, the memory 310 stores a single clock parameter whichis half the period of the clock's cycle. In another embodiment, thesingle clock parameter stored is the period of the clock's cycle. Inanother embodiment, the parameters stored are the period and the dutycycle of the clock.

A random jitter generator 302 randomly selects a jitter value within thestored jitter range (e.g., select a value between −N and +M). The randomjitter generator 302 generates a jitter clock value based on the readclock parameter and the randomly selected jitter value. In oneembodiment, the random jitter generator 302 generates the jitter clockvalue by summing the read clock parameter and the randomly selectedjitter value. In other embodiments, the jitter clock value can begenerated by combining the clock parameter and the randomly selectedjitter value in a different manner (e.g., multiply, divide, or calculatethe difference between the clock parameter and the jitter value). Therandom jitter generator 302 outputs the jitter clock value via output316 for storing the jitter clock value in the accumulator 304.

Each accumulator 304 is a storage component that stores a jitter clockvalue. An accumulator 304 receives the jitter clock value from thecorresponding random jitter generator 302 through input 318A and storesthe value. The accumulator 304 outputs the jitter clock value to itsrespective comparator 308 via output 320 and also outputs the jitterclock value to the minimum operator 306 via output 322.

The minimum operator 306 receives the system fast clock 311 and during acycle of the system fast clock, the minimum operator 306 selects thelowest jitter clock value from the values stored by the accumulators304. The minimum operator 306 receives the jitter clock values stored bythe accumulators 304 via inputs 324. From the received jitter clockvalues, the minimum operator 306 selects the jitter clock value with thelowest value. The minimum operator 306 outputs the lowest jitter clockvalue to the comparators 308 via outputs 326.

A comparator 308 determines whether to output an edge on its respectiveclock signal 312. Each comparator 308 receives via input 328 the lowestjitter clock value output by the minimum operator 306. Each comparator308 also receives via input 330 the jitter clock value stored by itsrespective accumulator 304. A comparator 308 compares the jitter clockvalue stored by the accumulator 304 to the lowest jitter clock value. Ifthe values are not the same, the comparator 308 does not change thestate of its respective clock signal 312 (does not output an edge). Onthe other hand, if the values are the same, the comparator 308 changesthe state of the clock signal 312. If the clock signal 312 was at a lowstate, the comparator 308 outputs a high state (a rising edge) for theclock signal 312. However, if the clock signal was at a high state, thecomparator outputs a low state (a falling edge).

In one embodiment, when the values are the same, instead of changing thestate of the clock signal 312 during the current system fast clockcycle, the comparator 308 may wait for one or more additional systemfast clock cycles to pass before changing the state of the clock signal312. The comparator 308 may wait to change the state because it is notthe right time or because certain conditions have not been met. Forexample, if a test bench running the emulator 110 is not ready to outputthe state change, the comparator 308 will wait until the test bench isready.

Further, each comparator 308 calculates the difference between the valuestored by the respective accumulator 304 and the lowest jitter clockvalue. A comparator 308 outputs the calculated difference via output 332and stores the difference in the respective accumulator 304 as the newjitter clock value. The new jitter clock value replaces the valuepreviously stored by the accumulator 304. In another embodiment, insteadof comparing the jitter clock value stored by the accumulator 304 to thelowest jitter clock value for determining whether to generate an edge,the comparator 308 calculates the difference between the accumulatorvalue and the lowest jitter clock value and determines to generate anedge if the calculated difference is equal to zero.

If the value stored by the comparator 308 in the accumulator 304 iszero, the respective random jitter generator 302 generates a new jitterclock value for the accumulator 304. The random jitter generator 302reads a clock parameter of the respective clock signal 312 from thememory 310. In the embodiment where the memory 310 stores at least twoclock parameters for each clock signal 312, the random jitter generator302 reads a clock parameter different from the clock parameterpreviously read. For example, if the random jitter generator 302previously read the width of the positive portion of the clock cycle,the random jitter generator 302 reads the width of the negative portion.The random jitter generator 302 generates a new jitter clock value bysumming the read clock parameter and a jitter value randomly selectedfrom the jitter range. The random jitter generator 302 stores the newjitter clock value in the accumulator 304 replacing the previouslystored value.

During the next cycle of the system fast clock, the accumulators 304output their stored jitter clock value, the minimum operator 306 selectsthe lowest jitter clock value, the comparators 308 determine whether tooutput an edge, and new jitter clock values are stored in theaccumulators 304. Hence, the process repeats during each cycle of thesystem fast clock. Further, each time a clock parameter is read from thememory 310, jitter is incorporated into the parameter by a random jittergenerator 302. In one embodiment, the process is repeated only when anew edge is required on any of the clocks.

In one embodiment, one or more memories are used to store theaccumulators 304. A memory storing the accumulators 304 can be memory310.

In one embodiment, the operations described in FIG. 3 are performed by aprocessor or a micro-controller. In this embodiment, the components 302,304, 306, and 308 are instructions that cause the processor to read theclock parameters from a memory 310, select a clock jitter value, storethe accumulation, calculate a minimum, perform the comparison, issueclock edges and update the accumulators. The order of instructions canbe different.

FIG. 4 is a timing diagram 400 illustrating clock signal 312A, clocksignal 312B, and system fast clock 311 during operation of the clockgenerator 300 according to one embodiment. The timing diagram 400illustrates seven cycles of the system fast clock 404A, 404B, 404C,404D, 404E, 404F, 404G. For this example, assume that clock signals 312start at a low state and the memory 310 stores a single clock parameterfor each clock signal 312. For signal 312A the memory 310 stores a clockparameter of ‘55’ and for signal 312B the memory 310 stores a clockparameter of ‘60.’ Further, assume that the jitter range for signal 312Ais ‘±5’ and that the jitter range for signal 312B is ‘±3.’

When the clock generator 300 is initialized the clock generator 300reads the clock parameter of ‘55’ for clock signal 312A from the memory310 and randomly selects a jitter value of ‘+4’ from the jitter range‘±5.’ The clock generator 300 sums the clock parameter and the jittervalue and stores the result ‘59’ in accumulator 304A. Additionally, theclock generator 300 reads the clock parameter of ‘60’ for clock signal312B from the memory 310 and selects a jitter value of ‘−3’ from thejitter range ‘±3.’ The clock generator 300 sums them and stores theresult ‘57’ in accumulator 304B.

During clock cycle 404A, the clock generator 300 determines thataccumulator 304B includes the lowest value of ‘57’ and as a resultoutputs rising edge 406 for clock signal 312B. The clock generator 300reads the clock parameter of ‘60’ for clock signal 312B from the memory310 and randomly selects a jitter value of ‘−2’ from the correspondingjitter range. The clock generator 300 sums the clock parameter andjitter value and stores the result ‘58’ in accumulator 304B replacingthe previously stored value. Additionally, clock generator 300determines the difference between the value stored by accumulator 304A‘59’ and the lowest value ‘57.’ The clock generator 300 stores theresult ‘2’ in accumulator 304A.

During cycle 404B, the clock generator 300 determines that accumulator304A includes the lowest value of ‘2’ and as a result outputs risingedge 408 for clock signal 312A. The clock generator 300 reads the clockparameter of ‘55’ for clock signal 312A from the memory 310 and randomlyselects a jitter value of ‘+1.’ The clock generator 300 sums the clockparameter and the jitter value and stores the result ‘56’ in accumulator304A. Additionally, clock generator 300 determines the differencebetween the value stored by accumulator 304B ‘58’ and the lowest value‘2.’ The clock generator 300 stores the result ‘56’ in accumulator 304B.

During cycle 404C, the clock generator 300 determines that accumulators304A and 304B both include the lowest/same value of ‘56.’ As a result,the clock generator 300 outputs falling edge 410 for clock signal 312Aand falling edge 412 for clock signal 312B. The clock generator 300reads the clock parameter of ‘55’ for clock signal 312A from the memory310 and randomly selects a jitter value of ‘+2’ from the correspondingjitter range. The clock generator 300 sums the clock parameter and thejitter value and stores the result ‘57’ in accumulator 304A.Additionally, the clock generator 300 reads the clock parameter of ‘60’for clock signal 312B from the memory 310 and randomly selects a jittervalue of ‘−2’ from the corresponding jitter range. The clock generator300 sums the clock parameter and the jitter value and stores the result‘58’ in accumulator 304A.

During clock cycle 404D, the clock generator 300 outputs rising edge 414for clock signal 312A because accumulator 304A includes the lowest valueof ‘57.’ The clock generator 300 stores a jitter clock value of ‘60’ inthe accumulator 304A based on the sum of the clock parameter ‘55’ readfrom the memory 310 and a randomly selected jitter value of ‘+5.’Additionally, the clock generator 300 stores a jitter clock value of ‘1’in accumulator 304B because it is the difference between the valuepreviously stored by accumulator 304B ‘58’ and the lowest value ‘57.’

During clock cycle 404E, the clock generator 300 outputs rising edge 416for clock signal 312B because accumulator 304B includes the lowest valueof ‘1.’ The clock generator 300 stores a jitter clock value of ‘59’ inthe accumulator 30BA based on the sum of the clock parameter ‘60’ and arandomly selected jitter value of ‘−1.’ Additionally, the clockgenerator 300 stores a jitter clock value of ‘59’ in accumulator 304Abecause it is the difference between the value previously stored byaccumulator 304A ‘60’ and the lowest value ‘1.’

During clock cycle 404F, the clock generator 300 outputs falling edge418 for clock signal 312A and falling edge 420 for clock signal 312Bbecause accumulator 304A and 304B both include the lowest value of ‘59.’The clock generator 300 stores a jitter clock value of ‘50’ inaccumulator 304A (sum of the clock parameter ‘55’ and randomly selectedjitter value of ‘−5’). The clock generator 300 also stores a jitterclock value of ‘63’ in accumulator 304B (sum of the clock parameter ‘60’and randomly selected jitter value of ‘+3’). During clock cycle 404G,the clock generator 300 outputs rising edge 422 for clock signal 312Abecause accumulator 304A includes the lowest value of ‘50.’

The table below illustrates the values stored by each accumulator 304 atthe start of each system fast clock cycle 404 according to the exampleof FIG. 4. A star indicates that a new value was obtained from therandom jitter generator 302.

Cycle Cycle Cycle Cycle Cycle Cycle Cycle 404A 404B 404C 404D 404E 404F404G Accumulator 59* 2 56* 57* 60* 59  50* 304A Accumulator 57* 58* 56 58* 1 59* 63* 304B

As can be seen in the timing diagram 400, by including jitter, thecycles of the clock signals 312 will vary. For example, for clock signal312A, there are two system fast clock cycles (cycles 404B and 404C)between rising edges 408 and 414. However, between rising edges 414 and422, there are three system fast clock cycles (cycles 404D-404F). Ifjitter was not included, all of the cycles of each clock signal 312would be the same. The jitter and variation in the clock signals 312allows the emulator 110 to properly emulate, for example, clock domaincrossings in the DUT. One may also notice that without jitter the firstclock edge would have been on clock signal 312A as it has a lower value‘55’ than the clock signal 312B with a value ‘60’. As the edge on clocksignal 312B comes first, it may expose design bugs that would not havebeen exposed without this invention

FIG. 5 is a flow chart illustrating a process of emulating multipleclock signals of a DUT, according to one embodiment. Other embodimentscan perform the steps of FIG. 5 in different orders. Moreover, otherembodiments can include different and/or additional steps than the onesdescribed here.

Assume for purposes of this example the emulator 110 stores clockparameters for the clock signals of the DUT. Additionally, assume thatthe emulator 110 stores a jitter range for each clock signal thatrepresents the jitter values the clock signal can have.

The emulator 110 generates 502 a jitter clock value for each clocksignal. To generate a jitter clock value for a clock signal, theemulator 110 identifies a clock parameter stored for the clock signaland sums the clock parameter with a jitter value randomly selected fromthe jitter range of the clock signal. The emulator 110 determines 504whether a cycle of a system fast clock has started. If a system fastclock cycle has not started, the emulator 110 waits 506 for the start ofthe next the system fast clock cycle.

When a system fast clock cycle starts, the emulator 110 determines 508the lowest value from the generated jitter clock values. The emulator110 outputs 510 an edge on clock signal having the lowest jitter clockvalue.

The emulator 110 generates 512 a new jitter clock value for each clocksignal. If a clock signal did not have the lowest jitter clock value,the emulator 110 determines for the clock signal the new jitter clockvalue to be the difference between the jitter clock value previouslygenerated for the clock signal and the lowest value. If a clock signalhad the lowest jitter clock value, the emulator 110 determines for theclock signal the new jitter clock value to be the sum of a clockparameter stored for the clock and a jitter value randomly selected fromthe jitter range of the clock signal. Steps 504-512 are then repeatedfor the new generated jitter clock value.

Computing Machine Architecture

Turning now to FIG. 6, it is a block diagram illustrating components ofan example machine able to read instructions from a machine-readablemedium and execute them in a processor (or controller). Specifically,FIG. 6 shows a diagrammatic representation of a machine in the exampleform of a computer system 600 within which instructions 624 (e.g.,software or program code) for causing the machine to perform (execute)any one or more of the methodologies described with FIGS. 1-5. Thecomputer system 600 may be used for one or more of the entities (e.g.,host system 110, emulator 120) illustrated in the emulation environment100 of FIG. 1.

The example computer system 600 includes a hardware processor 602 (e.g.,a central processing unit (CPU), a graphics processing unit (GPU), adigital signal processor (DSP), one or more application specificintegrated circuits (ASICs), one or more radio-frequency integratedcircuits (RFICs), or any combination of these), a main memory 604, and astatic memory 606, which are configured to communicate with each othervia a bus 608. The computer system 600 may further include graphicsdisplay unit 610 (e.g., a plasma display panel (PDP), a liquid crystaldisplay (LCD), a projector, or a cathode ray tube (CRT)). The computersystem 600 may also include alphanumeric input device 612 (e.g., akeyboard), a cursor control device 614 (e.g., a mouse, a trackball, ajoystick, a motion sensor, or other pointing instrument), a storage unit616, a signal generation device 618 (e.g., a speaker), and a networkinterface device 620, which also are configured to communicate via thebus 608.

The storage unit 616 includes a machine-readable medium 622 which storesinstructions 624 (e.g., software) embodying any one or more of themethodologies or functions described herein. The instructions 624 (e.g.,software) may also reside, completely or at least partially, within themain memory 604 or within the processor 602 (e.g., within a processor'scache memory) during execution thereof by the computer system 600, themain memory 604 and the processor 602 also constituting machine-readablemedia. The instructions 624 (e.g., software) may be transmitted orreceived over a network 626 via the network interface device 620.

While machine-readable medium 622 is shown in an example embodiment tobe a single medium, the term “machine-readable medium” should be takento include a single medium or multiple media (e.g., a centralized ordistributed database, or associated caches and servers) able to storeinstructions (e.g., instructions 624). The term “machine-readablemedium” shall also be taken to include any medium that is capable ofstoring instructions (e.g., instructions 624) for execution by themachine and that cause the machine to perform any one or more of themethodologies disclosed herein. The term “machine-readable medium”includes, but not be limited to, data repositories in the form ofsolid-state memories, optical media, and magnetic media.

As is known in the art, a computer system 600 can have different and/orother components than those shown in FIG. 6. In addition, the computersystem 600 can lack certain illustrated components. For example, acomputer system 600 acting as the emulator 120 may include one or morehardware processors 602, multiple storage units 616, a network interfacedevice 620, and multiple configurable logic circuits (as described abovewith reference to FIG. 1), among other components, but may lack analphanumeric input device 612 and a cursor control device 614.

Additional Configuration Considerations

Throughout this specification, plural instances may implementcomponents, operations, or structures described as a single instance.Although individual operations of one or more methods are illustratedand described as separate operations, one or more of the individualoperations may be performed concurrently, and nothing requires that theoperations be performed in the order illustrated. Structures andfunctionality presented as separate components in example configurationsmay be implemented as a combined structure or component. Similarly,structures and functionality presented as a single component may beimplemented as separate components. These and other variations,modifications, additions, and improvements fall within the scope of thesubject matter herein.

Certain embodiments are described herein as including logic or a numberof components, modules, or mechanisms, for example, as illustrated inFIGS. 1-6. Modules may constitute either software modules (e.g., codeembodied on a machine-readable medium or in a transmission signal) orhardware modules. A hardware module is tangible unit capable ofperforming certain operations and may be configured or arranged in acertain manner. In example embodiments, one or more computer systems(e.g., a standalone, client or server computer system) or one or morehardware modules of a computer system (e.g., a processor or a group ofprocessors) may be configured by software (e.g., an application orapplication portion) as a hardware module that operates to performcertain operations as described herein.

In various embodiments, a hardware module may be implementedmechanically or electronically. For example, a hardware module maycomprise dedicated circuitry or logic that is permanently configured(e.g., as a special-purpose processor, such as a field programmable gatearray (FPGA) or an application-specific integrated circuit (ASIC)) toperform certain operations. A hardware module may also compriseprogrammable logic or circuitry (e.g., as encompassed within ageneral-purpose processor or other programmable processor) that istemporarily configured by software to perform certain operations. Itwill be appreciated that the decision to implement a hardware modulemechanically, in dedicated and permanently configured circuitry, or intemporarily configured circuitry (e.g., configured by software (orcomputer program code)) may be driven by cost and time considerations.

The various operations of example methods described herein may beperformed, at least partially, by one or more processors, e.g.,processor 602, that are temporarily configured (e.g., by software) orpermanently configured to perform the relevant operations. Whethertemporarily or permanently configured, such processors may constituteprocessor-implemented modules that operate to perform one or moreoperations or functions. The modules referred to herein may, in someexample embodiments, comprise processor-implemented modules.

The one or more processors may also operate to support performance ofthe relevant operations in a “cloud computing” environment or as a“software as a service” (SaaS). For example, at least some of theoperations may be performed by a group of computers (as examples ofmachines including processors), these operations being accessible via anetwork (e.g., the Internet) and via one or more appropriate interfaces(e.g., application program interfaces (APIs).)

The performance of certain of the operations may be distributed amongthe one or more processors, not only residing within a single machine,but deployed across a number of machines. In some example embodiments,the one or more processors or processor-implemented modules may belocated in a single geographic location (e.g., within a homeenvironment, an office environment, or a server farm). In other exampleembodiments, the one or more processors or processor-implemented modulesmay be distributed across a number of geographic locations.

Some portions of this specification are presented in terms of algorithmsor symbolic representations of operations on data stored as bits orbinary digital signals within a machine memory (e.g., a computermemory). These algorithms or symbolic representations are examples oftechniques used by those of ordinary skill in the data processing artsto convey the substance of their work to others skilled in the art. Asused herein, an “algorithm” is a self-consistent sequence of operationsor similar processing leading to a desired result. In this context,algorithms and operations involve physical manipulation of physicalquantities. Typically, but not necessarily, such quantities may take theform of electrical, magnetic, or optical signals capable of beingstored, accessed, transferred, combined, compared, or otherwisemanipulated by a machine. It is convenient at times, principally forreasons of common usage, to refer to such signals using words such as“data,” “content,” “bits,” “values,” “elements,” “symbols,”“characters,” “terms,” “numbers,” “numerals,” or the like. These words,however, are merely convenient labels and are to be associated withappropriate physical quantities.

Unless specifically stated otherwise, discussions herein using wordssuch as “processing,” “computing,” “calculating,” “determining,”“presenting,” “displaying,” or the like may refer to actions orprocesses of a machine (e.g., a computer) that manipulates or transformsdata represented as physical (e.g., electronic, magnetic, or optical)quantities within one or more memories (e.g., volatile memory,non-volatile memory, or a combination thereof), registers, or othermachine components that receive, store, transmit, or displayinformation.

As used herein any reference to “one embodiment” or “an embodiment”means that a particular element, feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment.

Some embodiments may be described using the expression “coupled” and“connected” along with their derivatives. For example, some embodimentsmay be described using the term “coupled” to indicate that two or moreelements are in direct physical or electrical contact. The term“coupled,” however, may also mean that two or more elements are not indirect contact with each other, but yet still co-operate or interactwith each other. The embodiments are not limited in this context.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

In addition, use of the “a” or “an” are employed to describe elementsand components of the embodiments herein. This is done merely forconvenience and to give a general sense of the invention. Thisdescription should be read to include one or at least one and thesingular also includes the plural unless it is obvious that it is meantotherwise.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative structural and functional designs for theembodiments described herein. Thus, while particular embodiments andapplications have been illustrated and described, it is to be understoodthat the disclosed embodiments are not limited to the preciseconstruction and components disclosed herein. Various modifications,changes and variations, which will be apparent to those skilled in theart, may be made in the arrangement, operation and details of the methodand apparatus disclosed herein without departing from the spirit andscope defined in the appended claims.

What is claimed is:
 1. A non-transitory computer readable storage mediumstoring executable instructions for generating a plurality of clockssignals of a design under test (DUT), the instructions to configure anemulator to: identify a plurality of jitter clock values, each jitterclock value associated with a clock signal from the plurality of clocksignals of the DUT being emulated by the emulator and each jitter clockvalue determined based on a jitter value and a clock parameterassociated with the clock signal; identify, from the plurality of jitterclock values, a lowest jitter clock value; and output an edge on a clocksignal, from the plurality of clock signals, associated with the lowestjitter clock value.
 2. The non-transitory computer readable storagemedium of claim 1, wherein the instructions further configure theemulator to: randomly select the jitter value from a jitter rangeassociated with the clock signal.
 3. The non-transitory computerreadable storage medium of claim 1, wherein the clock parameterassociated with the clock signal is associated with a period of theclock signal.
 4. The non-transitory computer readable storage medium ofclaim 1, wherein the instructions further configure the emulator to:determine a jitter clock value from the plurality of jitter clock valuesby combining the jitter value and the clock parameter.
 5. Thenon-transitory computer readable storage medium of claim 1, wherein theinstructions further configure the emulator to: for each of theplurality of clock signals associated with a jitter clock value that isnot the lowest jitter clock value, maintain a state of the clock signal.6. The non-transitory computer readable storage medium of claim 1,wherein the instructions further configure the emulator to: for each ofthe plurality of clock signals associated with a jitter clock value:generate an additional jitter clock value for the clock signal based onwhether the jitter clock value is the lowest jitter clock value. Thenon-transitory computer readable storage medium of claim 6, wherein theinstructions further configure the emulator to: generate the additionaljitter clock value to be a difference between the jitter clock value andthe lowest jitter clock value in response to the jitter clock value notbeing the lowest jitter clock value.
 8. The non-transitory computerreadable storage medium of claim 6, wherein the instructions furtherconfigure the emulator to: generate the additional jitter clock value bysumming an additional clock parameter associated with the clock signaland an additional jitter value randomly selected from a jitter range, inresponse to the jitter clock value associated with the clock signalbeing the lowest jitter clock value.
 9. A non-transitory computerreadable storage medium storing executable instructions for generating aclock signal of a design under test (DUT), the instructions to configurean emulator to: obtain a clock parameter associated with the clocksignal of the DUT being emulated by the emulator; randomly generate ajitter value; and generate a jitter clock value for the clock signalbased on the clock parameter and the jitter value.
 10. Thenon-transitory computer readable storage medium of claim 9, wherein theinstructions further configure the emulator to: identify a plurality ofjitter clock values including the generated jitter clock value, each ofthe plurality of jitter clock values associated with a different clocksignal of the DUT; identify, from the plurality of jitter clock values,a lowest jitter clock value; and output an edge on a clock signalassociated with the lowest jitter clock value.
 11. The non-transitorycomputer readable storage medium of claim 9, wherein the clock parameterand the jitter value are combined to generate the clock jitterparameter.
 12. The non-transitory computer readable storage medium ofclaim 9, wherein the jitter value is generated by randomly selecting thejitter value from a jitter range associated with the clock signal. 13.The non-transitory computer readable storage medium of claim 9, whereinthe clock parameter associated with the clock signal is associated witha period of the clock signal.
 14. A clock generator emulated by anemulator, the clock generator comprising: a first storage configured tostore a first jitter clock value associated with a first clock signal ofa design under test (DUT) emulated by the emulator, the first jitterclock value determined based on a first jitter value and a first clockparameter associated with the first clock signal; a second storageconfigured to store a second jitter clock value associated with a secondclock signal of the DUT, the second jitter clock value determined basedon a second jitter value and a second clock parameter associated withthe second clock signal; a minimum operator configured to determine thatthe first jitter clock value is less than the second jitter clock value;and a comparator configured to output an edge on the first clock signalbased on first jitter clock value being less than the second jitterclock value.
 15. The clock generator of claim 14, further comprising anadditional comparator wherein: the minimum operator is furtherconfigured to output to the comparator and to the additional comparatorthe first jitter clock value based on the first jitter clock value beingless than the second jitter clock value; the comparator is furtherconfigured to: receive the first jitter clock value output by theminimum operator; read from the first storage the stored first jitterclock value; and output the edge of the first clock signal based on thefirst jitter clock value output by the minimum operator being the sameas the first jitter clock value read from the first storage; and theadditional comparator configured to: receive the first jitter clockvalue output by the minimum operator; read from the second storage thestored second jitter clock value; and maintain a state of the secondclock signal based on the first jitter clock value output by the minimumoperator being different than the second jitter clock value read fromthe second storage.
 16. The clock generator of claim 14, furthercomprising: a first random jitter generator configured to randomlyselect the first jitter value from a first jitter range associated withthe first clock signal; and a second random jitter generator configuredto randomly select the second jitter value from a second jitter rangeassociated with the second clock signal.
 17. The clock generator ofclaim 14, further comprising: a first random jitter generator configuredto: generate a third jitter clock value for the first clock signal basedon a third jitter value and a third clock parameter associated with thefirst clock signal; and store the third jitter clock value in the firststorage to replace the first jitter clock value. an additionalcomparator configured to: generate a fourth jitter clock value for thesecond clock signal based on a difference between the second jitterclock value and the first jitter clock value; and store the fourthjitter clock value in the storage component to replace the second jitterclock value.
 18. A method for generating a plurality of clocks signalsof a design under test (DUT), the method comprising: identifying, by anemulator, a plurality of jitter clock values, each jitter clock valueassociated with a clock signal from the plurality of clock signals ofthe design under test (DUT) being emulated by the emulator and eachjitter clock value determined based on a jitter value and a clockparameter associated with the clock signal; identifying, by the emulatorfrom the plurality of jitter clock value, a lowest jitter clock value;and outputting, by the emulator, an edge on a clock signal, from theplurality of clock signals, associated with the lowest jitter clockvalue.
 19. The method of claim 18, further comprising: for each of theplurality of clock signals associated with a jitter clock value that isnot the lowest jitter clock value, maintaining a state of the clocksignal.
 20. The method of claim 18, further comprising for each of theplurality of clock signals associated with a jitter clock value:generating an additional jitter clock value for the clock signal basedon whether the jitter clock value is the lowest jitter clock value.